Semiconductor laser

Abstract

A semiconductor laser includes a substrate having a principal surface; an active region disposed on the principal surface of a substrate, the active region including a quantum well structure, the active region having a top surface, a bottom surface facing the top surface, and side surfaces; an emitter region including a first semiconductor region of a first conductivity type on the top surface of the active region; and a collector region including a second semiconductor region of the first conductivity type on at least one side surface of the active region. The quantum well structure includes unit cells that are arranged in a direction of an axis intersecting the principal surface of the substrate.

What is claimed is: 
     
       1. A semiconductor laser comprising:
 a substrate having a principal surface; 
 a mesa structure disposed on the principal surface of the substrate, the mesa structure extending in a direction parallel to the principal surface, the mesa structure including an active region, the active region including a quantum well structure, the active region having a top surface, a bottom surface facing the top surface, and side surfaces; 
 a first cladding layer having a refractive index lower than that of the active region, the first cladding layer being in contact with the top surface of the active region, the first cladding layer having an opening; 
 a second cladding layer having a refractive index lower than that of the active region, the second cladding layer covering the bottom surface of the active region except for a bottom center portion thereof; 
 an emitter region including a first semiconductor region of a first conductivity type on the top surface of the active region, the first semiconductor region being in contact with the top surface inside the opening of the first cladding layer so as to supply carriers to the active region; 
 a collector region including a second semiconductor region of the first conductivity type on at least one side surface of the active region, the second semiconductor region being in contact with the at least one side surface so as to receive the carriers from the active region; and 
 a third semiconductor region of the first conductivity type on the bottom surface of the active region, the third semiconductor region being in contact with the bottom center portion of the bottom surface of the active region and the principal surface of the substrate, wherein 
 the active region emits light by the carriers transiting in the active region, 
 the collector region is separated from the emitter region by the first cladding layer being therebetween on the mesa structure, 
 the top surface and the bottom surface of the active region extend in planes parallel to the principal surface, and 
 the second semiconductor region is disposed on the at least one side surface of the active region and extends along a plane perpendicular to the principal surface of the substrate. 
 
     
     
       2. The semiconductor laser according to  claim 1 , wherein
 the quantum well structure includes a plurality of unit cells each having a first well layer, a second well layer, a first barrier layer, and a second harrier layer, 
 the first barrier layer separates the first well layer from the second well layer, and 
 the first well layer separates the first barrier layer from the second barrier layer. 
 
     
     
       3. The semiconductor laser according to  claim 2 , wherein
 the unit cells are arranged in the direction of an axis perpendicular to the principal surface, and 
 the first harrier layer has a thickness smaller than that of the second barrier layer. 
 
     
     
       4. The semiconductor laser according to  claim 2 ,
 wherein the first barrier layer is partially or completely doped with a dopant of the first-conductivity-type. 
 
     
     
       5. The semiconductor laser according to  claim 1 , further comprising a first electrode disposed on the first semiconductor region and a second electrode disposed on the second semiconductor region, wherein
 the first electrode is electrically connected to the first semiconductor region, and 
 the second electrode is electrically connected to the second semiconductor region. 
 
     
     
       6. The semiconductor laser according to  claim 5 , further comprising a third electrode disposed on a back surface of the substrate. 
     
     
       7. The semiconductor laser according to  claim 1 , wherein
 the quantum well structure provides an upper energy level and a lower energy level having an energy lower than that of the upper energy level, 
 the first semiconductor region includes a first semiconductor layer disposed on the top surface of the active region, and a second semiconductor layer disposed on the first semiconductor layer, 
 the first semiconductor layer is in contact with the top surface of the active region, 
 the first semiconductor layer has a conduction band energy higher than or equal to the upper energy level, and 
 the second semiconductor layer has a refractive index lower than an equivalent refractive index of the active region. 
 
     
     
       8. The semiconductor laser according to  claim 1 ,
 wherein the quantum well structure provides an upper energy level and a lower energy level having an energy lower than that of the upper energy level, 
 the second semiconductor region includes a first semiconductor layer disposed on the at least one side surface of the active region, and a second semiconductor layer disposed on the first semiconductor layer, 
 the first semiconductor layer is in contact with the at least one side surface of the active region, 
 the first semiconductor layer has a conduction band energy lower than or equal to the lower energy level, and 
 the second semiconductor layer has a refractive index lower than an equivalent refractive index of the active region. 
 
     
     
       9. The semiconductor laser according to  claim 8 ,
 wherein the quantum well structure further provides a relaxation energy level having an energy lower than that of the lower energy level, and 
 the first semiconductor layer has a conduction band energy lower than or equal to the relaxation energy level.